In such an apparatus, a piston is mounted so that it can move in a cylinder where it can be propelled, by the explosion of a mixture of air and a fuel gas injected, from a cartridge, into a combustion chamber and to drive an element, for example a fastener (a nail or an insert) or some other anchor. Apart form the bearing-against-something safety feature, that prevents any firing when the apparatus is not pressed against a receiving material, this kind of apparatus has a member, known as a cage, which, when the apparatus is pressed against something, and by way of a feeler that may be a moving insert guide, pushes a chamber sleeve tube back until it comes into abutment against the cylinder head that carries the spark plug intended to cause the explosion, so as to close the combustion chamber thus formed by this sleeve tube, the cylinder head and the crown of the piston. The cage and the sleeve tube may be connected to one another by screwing. Furthermore, the retreat of the cage is performed against the action of a return spring, to return the apparatus to the open position, also bearing against the piston cylinder.
Because the combustion chamber bears against the cylinder head, stressful pressures are exerted on the cylinder head when such an apparatus is operating, particularly since the surface portions of the cylinder head and of the cylinder face one another. Even if, in other apparatuses, these detrimental pressures are no longer exerted on the cylinder head, they are transferred to the cage, which is not necessarily any better.